Passive infrared detectors focus radiation from an area to be monitored such that the presence and movement of a human, pet or similar object through a monitored space can be detected and preferably distinguished as such. In most systems, a Fresnel focusing arrangement, including any arrangement of lens or mirrors, is used to focus the infrared radiation emitted by the target onto a passive infrared detector for further processing.
To improve the response characteristics of the sensor, the Fresnel focusing arrangement can include any number of features such as multiple lenslets, in which each lenslet includes a focusing element to define an infrared beam that collectively covers a protected area or section thereof. The width of the infrared beams typically increase in size as an increasing function that is proportional to the distance from the sensor. This characteristic of the Fresnel focusing arrangement may make it difficult to distinguish between small pets located in a region close to the sensor, from a human located at a substantial distance from the sensor. For example, at distances closer to the sensor, the width of the infrared beams are narrow and as such, a small pet will produce a signal similar in level to a person detected at a substantial distance away from the sensor.
To address such problems, some systems have attempted to alter the collection features of the sensor. For example, some systems incorporate a Fresnel focusing arrangement that excludes the signals generated by small pets located in a region close to the sensor. The Fresnel focusing arrangement in this case can include a dead or reduced-capacity bottom zone for the sensor region closest to the sensor in order to shield signals from pets at this distance and thereby reduce false alarms caused by such pets. However, this fails to substantially address the problems created when tall or vertically mobile pets and typography allow pets to easily enter other, non-shielded zones.
Alternatively, a single passive infrared detector sensor may be used that includes a number of gaps between the sensing beams or zones. These sensing beams or zones are spaced such that a small pet must enter a dead zone associated with the gaps as they move across the space. In contrast, a person is a much larger and taller object and will therefore, produce a signal regardless whether he is standing at a position which at floor lever, is in a dead zone. With this arrangement, a pet produces a pulse signal with a very low component when the pet is in the dead zone, whereas a person, although producing a pulsed signal, produces a signal which is much more constant and can be easily distinguished from a pet. However, this also fails to substantially address the problems created when vertically mobile pets and topology allow pets to easily enter high areas in close proximity to the sensor which can bridge two active zones much in the manner as a person.
In yet another attempt to address such problems, two different types of sensors can be used to help distinguish between a person and pet. For example, these systems can include a microwave sensor in combination with a passive infrared sensor. However, this type of dual technology sensor greatly increases the cost and complexity of the system. In yet another similar attempt to address such problems, a two-element, passive infrared detector can be used to help distinguish between a human and pet. The system can receive and process the signals from the two detectors to help distinguish between a person and pet. Typically one element is provided to detect radiation signals at lower or closer sensing ranges, and a second detector is provided to detect radiation signals at higher or further sensing ranges. A pet does not have the height to trigger both detectors. However, this also fails to substantially address the problems created when tall or vertically mobile pets and typography allow pets to easily enter high areas in close proximity to the sensor and as with other two sensor systems, the additional sensors greatly increases the cost and complexity of the system.